FIND A TEACHER!!!
ZonGuy, how long have you been playing the upright? I'm admittedly making some assumptions, but if you are having that kind of recurring pain, I would assume that you haven't been playing for very long and you are self taught (forgive me if I am mistaken!). I know lots of great electric bassists who have had similar pain when switching to upright. It is important to remember that the upright requires you to put weight into the strings (from your shoulder and triceps), not to press the strings or squeeze the neck, which can cause pain similar to yours. Perhaps you should try playing on a stool for a while, as this immediately helps to resolve this issue because you won't be worried about balancing the instrument.
Again, find a teacher!!! This issue can only truly be resolved in direct human contact by someone who can see what you're doing. But if this is not an immediate possibility, try a simple exercise: glissando up and down the G string, from half position to the end of the fingerboard. Keep your wrist flat, and don't let your thumb touch the back of the neck at all, and keep even weight on the string from all fingers (note the use of the word weight, and not the word pressure!). Be very deliberate and slow.
Best of luck alleviating this problem, and to reiterate what I've said twice already, find a good teacher!
